This invention relates to a method and apparatus for performing nuclear magnetic resonance (NMR) measurements, and more particularly, the invention relates to an arrangement for efficiently performing T1-based and T2-based measurements.
Nuclear magnetic resonance (NMR) measurements typically are performed to investigate properties of a sample. For example, an NMR wireline or logging while drilling (LWD) downhole tool may be used to measure properties of subterranean formations. In this manner, a typical NMR tool may, for example, provide a lithology-independent measurement of the porosity of a particular formation by determining the total amount of hydrogen present in fluids of the formation. Equally important, the NMR tool may also provide measurements that indicate the dynamic properties and environment of the fluids, as these factors may be related to petrophysically important parameters. For example, the NMR measurements may provide permeability and viscosity information that is difficult or impossible to derive from other conventional logging arrangements. Thus, it is the capacity of the NMR tool to perform these measurements that makes it particularly attractive versus other types of downhole tools.
Typical NMR logging tools include a magnet that is used to polarize hydrogen nuclei (protons) in the formation and a transmitter coil, or antenna, that emits radio frequency (RF) pulses. A receiver antenna may measure the response (indicated by received spin echo signals) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna.
There are several experimental parameters that may be adjusted according to the objectives of the NMR measurement and expected properties of the formation fluids. However, the NMR techniques employed in current NMR tools typically involve some variant of a basic two step sequence that includes a polarization time followed by an acquisition sequence.
During the polarization time (often referred to as a "wait time") the protons in the formation polarize in the direction of a static magnetic field (called B.sub.0) that is established by a permanent magnet (of the NMR tool). The growth of nuclear magnetization M(t) (i.e., the growth of the polarization) is characterized by the "longitudinal relaxation time" (called T1) of the fluid and its equilibrium value (called M.sub.0). When the specimen is subject to a constant field for a duration t.sub.p, the longitudinal magnetization is: ##EQU1##
The duration of the polarization time may be specified by the operator (conducting the measurement) and includes the time between the end of one acquisition sequence and the beginning of the next. For a moving tool, the effective polarization time also depends on tool dimensions and logging speed.
Referring to FIG. 1, as an example, a sample (in the formation under investigation) may initially have a longitudinal magnetization M.sub.Z 10 of approximately zero. The zero magnetization may be attributable to a preceding acquisition sequence (for example), as described below. However, in accordance with equation 1, the magnetization M.sub.Z 10 (under the influence of the B.sub.0 field) increases to a magnetization level (called M(t.sub.p (1))) after a polarization time t.sub.p (1)after zero magnetization. As shown, after a longer polarization time t.sub.p (2) from zero magnetization, the magnetization M.sub.Z 10 increases to an M(t.sub.p (2)) level.
An acquisition sequence begins after the polarization time. For example, an acquisition sequence may begin at time t.sub.p (1), a time at which the magnetization M.sub.Z 10 is at the M(t.sub.p (1)) level. At this time, RF pulses are transmitted from a transmitter antenna of the tool. The pulses, in turn, produce spin echo signals 16. A receiver antenna (that may be formed from the same coil as the transmitter antenna) receives the spin echo signals 16 and stores digital signals that indicate the spin echo signals 16. The initial amplitudes of the spin echo signals 16 indicate a point on the magnetization M.sub.Z 10 curve, such as the M(t.sub.p (1)) level, for example. Therefore, by conducting several measurements that have different polarization times, points on the magnetization M.sub.Z 10 curve may be derived, and thus, the T1 time for the particular formation may be determined.
As an example, for the acquisition sequence, a typical logging tool may emit a pulse sequence based on the CPMG (Carr-Purcell-Meiboom-Gill) pulse train. The application of the CPMG pulse train includes first emitting a pulse that rotates the magnetization, initially polarized along the B.sub.0 field, by 90.degree. into a plane perpendicular to the B.sub.0 field. A train of equally spaced pulses follows, whose function is to maintain the magnetization polarized in the transverse plane. In between the pulses, magnetization refocuses to form the spin echo signals 16 that may be measured using the same antenna. Because of thermal motion, individual hydrogen nuclei experience slightly different magnetic environments during the pulse sequence, a condition that results in an irreversible loss of magnetization and consequent decrease in successive echo amplitudes. This rate of loss of magnetization is characterized by a "transverse relaxation time" (called T2) and is depicted by the decaying envelope 12 of FIG. 1. This may be referred to as a T2-based experiment.
Measurements of T1 are typically made using a method known as saturation recovery. In this approach, longitudinal magnetization is first destroyed, then allowed to recover for a length of time, t.sub.p, at which point it is monitored, using a radio frequency pulse or sequence of pulses, and the signal recorded in a receiver. The signal amplitude is proportional to the recovered magnetization at time, t.sub.p. By repeating the measurement for different t.sub.p values, the magnetization recovery profile, Mz(t.sub.p), is sampled and may be analyzed to determine the longitudinal relaxation time T1. This may be referred to as a T1 based experiment. If a sequence of pulses such as the CPMG sequence is used to monitor the magnetization recovery at time, t.sub.p, the initial amplitude of the echo decay envelope represents Mz(t.sub.p), while the echo decay profile, 12, yields T2 information corresponding to this longitudinal magnetization, Mz(t.sub.p). Analysis of these experiments provides information concerning both T1 and T2.
In a CPMG pulse train with a spacing (called TE) between the pulses, applied to a sample containing a single type of fluid, an amplitude, A(n) of the nth echo may be described by the following equation: ##EQU2##
where t.sub.p is the polarization time.
The measured NMR signal, A(n), is governed by three quantities (M.sub.0, T1 and T2) that reflect physical properties of the fluids and the formation. The equilibrium longitudinal magnetization M.sub.0 is used to compute the total porosity of formation, as described by the following equation: ##EQU3##
where HI is the hydrogen index of the formation fluid, and K is a calibration factor that accounts for several tool and external parameters. Relaxation times are related to permeability of the formation as well as the fluid properties and may be used to identify hydrocarbon types. Water relaxation times increase with increasing pore size. Thus, short T1 or T2 times indicate bound water, while long T1 and T2 times are associated with free fluid. For hydrocarbons in water wet rocks, the T1 and T2 times are determined by viscosity. The T1 time increases with decreasing viscosity over the entire hydrocarbon range from bitumen to methane gas. The T2 time follows a similar trend for heavy and medium oils. For lighter hydrocarbons, diffusion effects reduce the T2 time. The effect is most significant for gas. Because of the wide range of pore sizes found in rock formations and the chemical complexity of typical oils, broad distributions of T1 and T2 times are usually observed. Whereas T2 distributions may be estimated by analysis of multi-exponential decays of CPMG echo amplitudes, it is necessary to perform several separate measurements using different polarization times t.sub.p, in order to properly characterize T1 distributions.
Typical logging tools that are based on the single antenna concept measure CPMG echo decay profiles using a fixed polarization time. In order to determine total porosity, a polarization time of at least three times the largest T1 time in the formation fluids may be used. In general, the T1 time is not known prior to logging, and thus, it is necessary to guess a reasonable value and set duration of the polarization time accordingly to this estimate. Overestimation of the T1 time results in inefficient logging, since the logging speed must be reduced accordingly. Underestimation of the T1 time leads to incomplete polarization and consequently, an underestimation of the total porosity.
In some cases it is considered expedient to obtain an experimental estimate of the T1 time. This may be done either to obtain an improved porosity estimate or for the purposes of hydrocarbon typing. The procedure for determining the T1 time with current tools includes repeating the standard NMR measurement (such as the CPMG pulse train) using different polarization times. As an example, after waiting for a first polarization time t.sub.p (1) see FIG. 1), a first CPMG pulse train may be applied to obtain the spin echo signals 16. In this manner, the initial amplitudes of the pulse train may be used to measure the magnetization level M(t.sub.p (1)), as an example. The CPMG pulse train effectively destroys the magnetization M.sub.Z (i.e., decreases the magnetization M.sub.Z 10 to approximately zero near the antenna). The next CPMG pulse train that is applied to obtain spin echo signals 18 to measure the M(t.sub.p (2)), level (for example) must first wait for a polarization time t.sub.p (2). This ensures that the polarization that was destroyed by the previous CPMG sequence is polarized by a predetermined duration in the intersection of the regions sensed by two consecutive CPMG sequences.
Therefore, measuring two points on the magnetization M.sub.Z curve 10 takes a time approximately equal to the summation of the times t.sub.p (1) and t.sub.p (2), a time that may consume several seconds, for example. Thus, using different polarization times may inevitably lead to a significant increase in the total logging time. Because of the typically limited number (due to the desire to decrease logging time) of different polarization times that may be used, it is rarely feasible to derive precise T1 times. Thus, analysis of the measurements is generally limited to simple comparisons of the separate measurements.
In the medical field, NMR measurements are performed with an "outside-in" device to investigate properties of a sample. U.S. Pat. No. 5,363,042 issued to Charles L. Dumoulin describes a magnetic resonance imaging system and method for measuring the T1 of moving blood. The '042 method takes advantage of the relative motion between the instrument and the specimen to reduce the total measurement time. The entire sample is fully polarized to its equilibrium magnetization M.sub.0. An inversion pulse nutates the spins so that they oppose the applied field. The spins start recovering from -M.sub.0 to +M.sub.0. The magnetization in a slice is imaged tp(i) seconds after the inversion where tp(1)&lt;tp(2)&lt;. . . tp(N). The slice is orthogonal to the direction of motion and all N images are taken at the same slice. The slice is selected by a pulse of magnetic field gradient in the direction of the motion. The spins in the slice are nutated by an RF pulse by 90 degrees to an orientation that is transverse to the applied static field. The precession of the spins in the transverse plane is recorded while an imaging sequence of gradient pulses is applied. Each image provides a point on the T1 recovery curve for every pixel where there is motion. The total measurement time is tp(N) plus the length of one imaging sequence. While the '042 method saves time over repolarizing and inverting after each measurement, the method is impractical in logging T1 of an earth formation or in measuring the T1 of a fluid flowing in a pipe. It would require the specimen to be in the uniform magnetic field for 5*T1+tp(N) seconds and would require the length of a magnet to be at least V*(5*T1+tp(N)) where V is the speed of relative motion between the apparatus and the specimen. Further, the '042 method would require the magnetic field to be uniform in the intersection of the specimen and the region of sensitivity of the RF coil or antenna, which is impossible with an "inside-out" NMR device used in well logging.
Thus, there is a continuing need to address one or more of the problems that are stated above.